1. Field of the Invention
The present invention relates to an input protection circuit, and in particular to an improved electrostatic discharge (ESD) input protection circuit which is capable of effectively discharging an electrostatic charge in order to prevent damage to internal circuit elements in an integrated circuit.
2. Description of the Conventional Art
FIG. 1 illustrates a conventional input protection circuit, which, as shown therein, includes a pad 1 coupling an input signal to an internal circuit, and a bipolar NPN shunt transistor 2 connected to the signal path between the pad 1 and the internal circuit with the base thereof connected to receive a substrate voltage (VBB) and the emitter thereof connected to ground voltage (Vss).
FIG. 2 illustrates the physical layout of the input protection circuit of FIG. 1. As shown therein, a semiconductor substrate includes a p well forming the base of the bipolar NPN shunt transistor 2, and an n+ region forming the collector of the transistor 2 which is connected between the pad 1 and the internal circuit, and another n+ region forming the emitter of the transistor 2 which is connected with the ground voltage (Vss). A field oxide region (FOX) isolates the n+ collector and emitter regions and overlies the p-well.
The operation of the conventional input protection circuit will now be explained with reference to FIGS. 1 and 2.
When an electrostatic charge is inputted via the pad, a break-down phenomenon occurs between the n+ well forming the collector and the p well forming the base for thus increasing the amount of the electrostatic charge upon the p well. Therefore, the transistor becomes forward biased into conduction and the electro-static charge is discharged (shunted) through the p well and the (n+) collector region to ground (VSS).
However, in the conventional input protection circuit, since each pad has connected thereto an input protection circuit, so that the electro-static discharge path is made short, when a large electrostatic charge is inputted, it is impossible to properly discharge the electro-static charge.